In my copending application Ser. No. 407,646, which has been entered into this application by reference, I describe how, in the prior art, in the use of vibration sources in seismic operations, the reference sweeps are sinusoidal signals of variable frequency, of selected frequency bandwidth, and of selected time duration. The need for a sinusoidal sweep signal of multibit amplitude stems from the general opinion widespread in the industry, that each step in the seismic process must be carried out with great fidelity. For example, seismic amplifiers are designed with floating point binary gain amplifiers, that record the signals to 16 bit precision.
In line with this attitude, it has been customary in vibrator operations to provide a sinusoidal reference sweep and a sinusoidal drive sweep to control the vibrator, with the ultimate object of having a sinusoidal output of the vibrator impressed on the earth. To this end, in the phase control of the vibrator, the output of the vibrator is compared in phase with the reference sweep to provide a phase error signal. The phase error signal is then used to shift the phase of the reference signal in a leading direction, or to vary the phase output of a sinusoidal output signal from a computer generator, etc. But the drive signal is maintained as a multibit sinusoidal signal.
Following a somewhat similar procedure, in my copending application Ser. No. 407,646, now U.S. Pat. No. 3,984,805 I teach how to drive the vibrator directly with the reference signal, and then use the output signal of the vibrator to shift the phase of the reference signal in a lagging direction, to provide a counterpart signal, with which to correlate the received signal.
I have dicovered that, in the use of low energy seismic sources, such as vibrator sources, where a large number of repititions of the source are added, or stacked, or composited, to provide a composited signal of sufficiently high signal to noise ratio, for further processing, that the conventional 16 bit signal is not required. As a matter of fact, where there is sufficient noise in the seismic signal, as shown by Fort et al in U.S. Pat. Nos. 3,881,166 and 3,883,725, a plurality of received signals digitized to 1 bit can be stacked and processed to provide results which are indistinguishable from results obtained with 16 bit signals, provided there are enough repetitions of the source.
Furthermore, I have discovered that it is possible to correlate a 1 bit digitized received signal, with a 1 bit digitized reference signal to provide a 1 bit digitized correlated signal, or correlogram, and then to add or composite a plurality of such 1 bit digitized correlograms, to provide a processed record of quality comparable to one processed throughout with multibit signals.
In view of this 1 bit processing of seismic signals, I have discovered that there is great redundency in the prior art processing of seismic signals, particularly for signals derived from low energy sources.
It is therefor an important object of this invention to use a vibrator that can be driven by a square wave sweep signal, or a sweep signal of 1 bit digitization, and can produce a seismic signal in the earth of square wave form, or a transmitted signal digitized to 1 bit.
It is a further object of this invention to digitize the received seismic signal to 1 bit amplitude, and to correlate the 1 bit received signal with the 1 bit transmitted signal, to provide a 1 bit correlogram, and to composite a plurality of such 1 bit digitized correlograms.
It is a still further object of this invention to generate a square wave reference sweep signal by creating a train of time-spaced pulses of selected values, of variable time spacing, and, responsive to this train of pulses to create a train of square waves of corresponding time duration values.
It is a still further object of this invention to generate from the output movement of the vibrator an electrical "transmitted" signal of square wave shape, or of 1 bit digitization.
It is a still further object of this invention to provide an impulsive force to a vibrator, and provide a vibrator in which the output motion of the vibrator, responsive to the impulsive force is completed in a time which is less than one-half of the shortest period of the train of pulses.